Method and device for the formation of pipes

ABSTRACT

A method for the manufacture of welded tubes is provided. The tube bodies are produced in a press or rounding apparatus and are fed to a welding machine, such as a laser or electron beam type welding machine. The tube is formed when the edges of the tube bodies are welded together. The method includes the transfer of the tube bodies from the press or rounding apparatus to the draw-in of the welding machine. One or more transfers modules, each having a separate drive, are used to convey the tube bodies. The one or more transfer modules, which are operated by a common control arrangement, are controlled so that the intermittently formed tube bodies are presented at the draw-in to the welding machine in as nearly continuous a sequence as possible.

CROSS REFERENCE TO RELATED APPLICATION

Applicant hereby claims foreign priority benefits under 35 U.S.C. § 119of Swiss Patent Application No. 1934/199 filed 22 Oct. 1999 and PCTApplication No. PCT/CH00/00454, filed 25 Aug. 2000, the disclosure ofwhich is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to methods for manufacturing welded tubes ingeneral, and to methods for manufacturing welded tubes that include thetransfer of tube bodies between a forming apparatus and a weldingmachine.

2. Background Information

There exists an increasing demand for a production plant for internalhigh pressure forming (“hydroforming”) of elongated, longitudinallyclosed, essentially tubular bodies for the production of shaped parts.These bodies may be configured as tubes or hollow bodies withrotationally asymmetrical, or only partly rotationally symmetrical,cross-sections, and may be used for the widest variety of purposes. Ifthe shaped parts fabricated by hydroforming are used, e.g., in a bodystructure of a car or lorry or other vehicle, the quality of the outerskin of the shaped parts is becoming increasingly important, as theseare increasingly used as body components and are therefore visible onthe finished body. Today, tubes with a length of approximately one (1)meter to four (4) meters and with diameter-to-thickness ratios of sixty(60) or more are being used for this purpose. However, it is likely thatthe lengths will continue to increase, and may exceed, e.g., five (5)meters, while the diameter-to-thickness ratios drop to forty (40) orless and also increase to one hundred (100), one hundred and fifty (150)or more. It is moreover likely that rotationally asymmetrical bodieswill also be used, e.g., bodies with elliptical or oval cross-sectionswith partially flattened or parallel side wall parts, and/or bodies withrectangular or triangular cross-sections. The cross-section is governedby the specific application, which may demand every possiblecross-sectional shape.

The initial material for such bodies to be processed by hydroforminginto shaped parts is a flat sheet-metal blank which constitutes theshell of the future body. The blank is formed in a press or roundingapparatus into an elongated tubular body, which is not longitudinallyclosed, as the corresponding sheet-metal edges are adjacent but are notyet joined together. Joining is done by welding with an energy beam suchas a laser or by some other suitable welding process while thelongitudinal edges are held in the butt position. The quality of theweld seam is subject to special requirements, e.g., as to its fluidtightness and elasticity during the ensuing hydroforming.

Generally speaking, elongated welded tubular bodies need to befabricated with a high quality welded joint and, depending on theapplication, with a high-quality outer skin, as scratches orindentations on the shaped parts are undesirable or unacceptable.Therefore the basic problem of the invention is to provide methods andapparatuses with which elongated tubular bodies, in particular with ahigh-quality outer skin, can be produced efficiently.

DISCLOSURE OF THE INVENTION

Hereinafter where mention is made of “tubes”, “tube bodies”, etc., theseare to be understood as signifying elongated tubular bodies withvariously configured cross-sections according to the remarks made above.

A method for the manufacture of welded tubes is provided. The tubebodies are produced in a press or rounding apparatus and are fed to awelding machine, such as a laser or electron beam type welding machine.The tube is formed when the edges of the tube bodies are weldedtogether. The method includes the transfer of the tube bodies from thepress or rounding apparatus to the draw-in of the welding machine. Oneor more transfer modules, each having a separate drive, are used toconvey the tube bodies. The one or more transfer modules, which areoperated by a common control arrangement, are controlled so that theintermittently formed tube bodies are presented at the draw-in to thewelding machine in as nearly continuous a sequence as possible.

Through the provision between the equipment for forming the tubebodies—(which may be a press that forms the tube bodies in a number ofbending steps, or a rounding apparatus)—on the one hand, and the weldingmachine on the other hand, of a number of transfer modules which areactuated by a common control so that the transfer modules can constitutea buffer between the equipment for forming the tube bodies and theactual welding machine, it becomes possible to harness an intermittenttube body production to a continuous welding operation. For example,three short tube bodies or one long tube body can be formedsimultaneously in a press, and by slowing and/or accelerating individualtube bodies in the transfer line, can be arranged into a series of tubebodies that is as continuous and as closely spaced as possible onrunning through the welding zone.

With a continuous feed of tubes, it is also possible to configure thetransfer line so that the tubes are presented in the welding zone for anoptimal length of production run.

In accordance with a further aspect, an effort is made to transfer thetube bodies in such a way that their outer skin remains free fromdamage, and also in particular, that correct orientation of the edges ispreserved during transfer, if need be with an initial squeezing of thetube body with a view to a subsequent closure of the gap in the weldingarea. Squeezing of the tubes may be advisable because after the plasticforming of the blank into a tube there is an elastic spring-back whichopens up the gap between the edges somewhat. Also, the gap may be narrowenough or not quite narrow enough, depending on the method used forforming the blank. Furthermore, the use of different sheet-metal alloysand/or thicknesses on one and the same plant can result in differentgaps, and this may need correcting in the subsequent processing of thetubular body.

The present method provides substantially damage-free transfer betweenthe press or rounding apparatus and the welding machine by carrying thetube bodies on the inside of the tube during the transfer.

Supporting the bodies from inside protects the outer skin from damage.Means for locating the edges during transfer can also operateinternally, or on the edges themselves. Such locating means, e.g., inthe form of internally acting rollers and/or leaf spring devices actingon the edges, are advantageous in bringing the tube body to the draw-into the welding machine ready orientated, so that at that point onlyslight adjustments of alignment are necessary in addition to the closureof the tubular body. It is also preferred that there be a facility forpartially closing the gap in the body while transfer is still takingplace, preferably by externally acting drive rollers whose mutualposition is adjustable.

It is also desirable to ensure smooth, e.g., jolt-free, run-in of thetube bodies at the draw-in to the welding machine, or as they are passedfrom the transfer apparatus to the welding machine; even if the transferapparatus does not assist further closure of a tube to be supplied whichis inherently too wide open. It should be possible for the weldingmachine to handle the widest possible variety of tube dimensions and gapwidths. Furthermore, it is advantageous that the quality of the outerskin of the tube should not be impaired by this operation.

The present method utilizes a draw-in means, in particular draw-inrollers, which receive the tube body in an open position of the draw-inmeans and then close to a predetermined setting to reduce the gapopening of the body and preferably to contact the body so as to advanceit. The draw-in means move back to the open position after the body haspassed through.

As the draw-in means are in an open position when they receive the tubebody, abrupt impact of the body on these means, possibly damaging theouter skin, does not occur. The closing of the draw-in rollers to thepredetermined setting, triggered by a tube body sensor, can be effectedrelatively gently so that the draw-in means or rollers, which arepreferably plastic-coated or made of plastic material, do no damage tothe outer skin of the body. In seizing the tubular body the draw-inmeans partially close the body while advantageously applying the forceneeded to propel the body through further stations of the weldingmachine as far as the next propelling means in the conveying direction.

In the manufacture of welded tubes, it is preferable to at least partlyclose the gap in the tube bodies before they reach the welding tool;advantageously, this should be done without damaging the outer skin ofthe bodies. The present method accomplishes this by at least largelyclosing the open body edges in the welding machine using a tool actingexternally on the body with a predetermined setting. Preferably, thetool has a floating mount and/or assumes a basic position through springforce. In these embodiments, because the tool acting externally on thetube body to close the edges has a floating mount, it is able to followany movements of the bodies about their longitudinal axis, and thislargely excludes the possibility of damage to the outer skin of thebody.

The tool is preferably adjustable over the circumference of the tubebody, and of course adjustable in such a way that the object of largelyclosing up the edges is realized through a continuous, narrowingsqueezing together of the body. This squeezing together is preferablyperformed against a tapering element that sets the edge-closure byplunging into the body, travelling with the body if need be, and (if so)provided with its own drive if need be. The edges must bear on thiselement, though not with large transverse forces. As the wall thicknessof the tube bodies is variable, the transverse forces must also beadjustable, which is realized by setting the tool to the taperingdiameter required. The tool may apply pressure to the body, e.g., bymeans of pneumatic cylinders.

At this juncture, a point should be noted that applies in principle toall stations in the welding plant: as the tube runs through, its shapechanges in a way that is non-uniform over its length. If, say, themiddle of a long tube is in the tool described above, a portion of theleading end is already welded, while the trailing end has a fully opengap. If the reshaping from front to rear is linear, it is so only bychance; the tube warps over its length, which adversely affects theposition of the edges to be welded. What is more, the warping changes aswelding progresses. Also, the warping is dependent on, e.g., the length,material and thickness of the tube. In other words, the tube is “alive”during processing, which may in particular compromise the edge positionthat is required for welding.

Another issue that must be addressed in the manufacturing of tubes isthat the edges to be welded must be guided to a correct position forwelding. The welding requires an edge closure with an extremely smalltolerance, which in the case of laser welding for example amounts toonly 0.08 mm.

The present method addresses this issue by utilizing a closing tool withone or more, preferably exchangeable, modules. Each module has rollersthat are arranged in a ring and are adapted to the tube cross-section.At least one of the rollers is adjustably mounted so that, over itsrange of adjustment no damage is caused to the tube body, especially tothe outer skin thereof, by its action on the body, owing to the changein position.

Through provision in the welding zone of a welding tool formed frommodules with rings of rollers, the requisite number of rollers can beprovided for each sheet-metal thickness and grade. These rollers may bedriven, and have the particular function of setting a defined edgeposition during welding.

Preferably, spring-mounted lower rollers may be provided to effect asqueezing together of the tube body edges to close the gap as nearlycompletely as possible, but at least to within the range of tolerancefor laser welding. Squeezing the edges together evens out cuffingvariations or waviness of the edges, but on the other hand it usuallycauses V-shaped splaying of the edges, which may be undesirable. Thiscan be prevented by the upper rollers, which guide and/or press theedges so that the edge faces are in contact with each other. Thepreferred mounting for the upper and lower rollers is one which preventsthe rims of the waisted rollers from leaving impressions in the tubebody surface.

By providing a plurality of modules, various actions can be performed onthe tube body by means of the rollers of these modules and a goodadaptation can be made to tubes that are difficult to Weld (owing tomaterial, dimensions, surface coating) by adding or removing modules toobtain an optimal effect on the tube body.

After welding, there is usually an inspection of the welded joint orseam quality. It is preferred that a drive be provided, preferably inthe form of a further roller ring module, to advance the tube into thezone where the (usually optical) inspection of seam quality takes place,so that the tube is guided and driven in this area also, this beingbeneficial for the inspection, which is sensitive to vibrations, i.e.reacts to positional fluctuations (positional variations) of the seam.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail by way ofexample, with the aid of the drawings.

FIG. 1 shows a schematic three-dimensional representation of a weldingplant.

FIG. 2 is a frontal view of the feed through opening of a module of thetransfer apparatus.

FIG. 3 is a frontal view of the feed through opening of a module of thedraw-in device of the welding machine.

FIG. 4 is a frontal view of the feed through opening of the tool.

FIG. 5 is a frontal view of the feed through opening of a module of thewelding tool.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a welding plant 1 with a transfer apparatus 2 and a weldingmachine in a highly simplified three-dimensional view. The methodsemployed and the apparatuses will be described in detail with referenceto FIG. 1. Individual methods and/or devices are shown in more detail inFIGS. 2 to 5, and will be described with the aid of these figures. Thewelding plant should preferably be used for welding tubes that areintended for subsequent hydroforming and end up as shaped parts. Thesetubes, or the shaped parts produced from them, are used, e.g., inmotor-vehicle construction. The outer skin of the tube bodies, usuallyconsisting of a coating, must not be harmed, so that the coating remainsintact on the shaped part.

In FIG. 1, the reference numeral 2 denotes the general arrangement of atransfer apparatus. The transfer apparatus of the welding plant bringsindividual tube bodies from a press (not shown) or rounding apparatus(not shown)—known facilities which form tube bodies from flatsheet-metal blanks—to the actual welding machine. The transfer apparatus2 is formed from individual modules, only two such modules 3 and 4 beingshown in the drawing. Each of the modules is designed as a separate unitto convey tube bodies with an independent drive. To convey the tubebodies from the press or rounding apparatus to the welding machine,several such modules, e.g., five, are usually coupled together tojointly form a transport path for the bodies. In FIG. 1 the coupling issuggested by elements 13 not shown in detail. Each of the modules may,e.g., have feet with rollers (also suggested only) so that individualmodules, or a number of modules, can easily be taken out of thetransport path. The figure also shows two bodies 17 and 18 beingconveyed over the conveyor path formed by the modules to the weldingmachine. The individual drives of the modules are controlled by a commoncontrol 90 whose presence is only suggested in the figure and whoseconnection to the module drives (which are not shown) is representedmerely by lines symbolizing electrical leads. Such a control can beconstructed in the conventional way by an expert in the field ofcontrols. By means of the individual modules, each module beingseparately operable and/or individually adjustable in its speed profilefor rate of feed, the tube bodies can be buffered before reaching theactual welding machine, so that a transition can be made fromintermittent operation, such, as prevails at the press or roundingapparatus (also referred to as a forming apparatus), to continuousoperation, as required for the welding machine. The length of thetransfer apparatus is determined by the number of modules to allow thevarious operating speeds and the intermittent press operation to beadapted to the continuous operation of the welding machine. For example,five modules can be connected together to make a transfer apparatuswhich has a length of five meters and delivers the bodies comingintermittently from the press to the welding machine as a series spacedat regular and very short intervals. The press may produce differentbody lengths, e.g., a series of three short tubes or one long tube, andthese tubes may be buffered accordingly in the transfer apparatus. Otherconfigurations are also possible.

In the example shown, the tube bodies are conveyed in the modules, ortransfer apparatus, on a carrier plate 8 projecting downwards from anupper carrier 7 of each module and engaging in the opened body. In FIG.1 the carrier plate is shown as if it were transparent, in order toreveal tube body parts and drive rollers located behind it. FIG. 2 is aview from in front, looking the opposite way to the conveying direction,of the module 4 of the transfer apparatus, in which the plate 8 engagingin the body 18 is again visible. The plate 8 preferably extends over thewhole length of the module, so that when modules are coupled together acontinuous plate 8 for the tube bodies is formed. The carrier 7 and theremaining elements of each module are arranged on corresponding supportframes which form a rigid unit for each module, and on which the feet(not shown in detail) of each module are also arranged. FIG. 2 showsthat rollers 15 and 16 on which the tube body 18 and of course furtherconveyed bodies run are arranged on the plate 8 inside the body 18.Instead of the dome-shaped rollers shown, rollers standing outhorizontally from the plate, or rails on which the tube bodies run,could of course be provided. In any case, the bodies are preferablysupported by internal elements, as the intention is to keep the outerskin of the bodies as free as possible from scratches, kinks or otherdamage. The plate 8 also serves as centering means for the bodies,keeping the position of their edges essentially constant duringtransport in the transfer apparatus, and preventing rotational drifting.Special centering means can also be provided on the plate 8, such asleaf springs projecting slightly from the plate which act on the edgesof the bodies to keep them in as uniform a position as possible. Thesprings may also serve to prevent contact of the body edge over thewhole length of the plate 8, and thus to reduce friction duringconveying. The drive for conveying the tube bodies is preferably formedby rollers 9 to 12 which are driven by motors in the drive gearboxes 5and 6 of the module, the drive speed being determined by the control 90,as previously explained. The rollers, which, are preferably verticallyarranged, i.e. the rollers 10 and 11 of FIG. 2 for example, consist of aplastic material preventing the outer skin of the body from beingscratched on being driven by the rollers. Preferably, the gap “d”between the rollers is adjustable so that the modules of the transferapparatus can be set to different body diameters, as suggested in FIG.2, which shows a spindle 14 with opposing threads for adjusting the gapbetween the rollers. In addition to the body 18 drawn in the figure, twofurther body diameters are shown in broken lines. These represent, e.g.,the smallest and largest bodies that can be conveyed by the transferapparatus. The presence of a body in each module can be detected by asensor, suggested in FIG. 2 only.

The tube body having been conveyed by the transfer modules reaches thepoint of entry to the actual welding machine. Here a draw-in device isshown which in the illustrated example comprises the rollers 22 and 23.The rollers on one side are drivable in rotation to pick up the bodiesfrom the transfer apparatus and convey them through the welding machine.FIG. 3, in another schematic view, shows the body 19 being acted on bythe draw-in rollers 22 and 23. These rollers are driven by drive motors24 and 25, and likewise consist of plastic material or have a plasticcoating, again in order not to damage the outer skin of the body at thispoint. The rollers also assume the function of a first reduction stagefor the gap “d1” in each body. This involves parting the rollers in anopen position at a distance greater than the diameter of the bodyexiting from the transfer apparatus. Thus the rollers are moved awayfrom the body, as indicated by the arrows in FIG. 3, to accept theindividual body between the rollers without contact with the rollers.This avoids the rollers coming abruptly into contact with the tubebodies and partially closing them forcibly, which could cause damage tothe outer skin of the bodies. A sensor 92 above the rollers, suggestedin FIG. 1, detects the entry of a body between the rollers. After entry,the rollers are brought into contact with the tube, closing it, andsetting the gap “d1” to a predetermined size which is dictated interalia by the ensuing tool. This involves moving the rollers towards eachother as indicated by the arrows in FIG. 3. For this purpose the rollersmay be arranged on swivel arms (not shown) which are moved, e.g.,pneumatically or hydraulically and carry the rollers with their drivemotors 24 and 25.

The body, with its gap now reduced to a predetermined size, is then fedto a tool which effects a continuous, narrowing, further closure of thegap in the tube. For this purpose a limit stop 31 may be provided whichplunges into the tube and which consists, e.g., of individual elements32 that project into the body and circulate at the same speed as that atwhich the body is conveyed. These elements converge to a pointed tip, sothat by arranging the entire limit stop 31 so that it slopes upwardswith respect to the longitudinal axis of the tube, the result is a limitstop that becomes narrower as it travels along with the tube. The tubebody is externally pressed by tools 33 against the limit stop, or, inthe absence of such a stop, against the other edge, so that eithercontact occurs or only a small tolerance remains between the stop andthe body edges. In the illustrated example the tool has roller bars 88with a large number of rotatable rollers arranged thereupon that act onthe tube. The roller bars 88, which are arranged on brackets 34 and 36,can be adjusted in their position and, e.g., pressed against the tubepneumatically by adjusting means 35 and 37 so that the tube bears on thetapering limit stop. The tools 33 are preferably arranged to float sothat they are able to follow any movement of the tube about itslongitudinal axis. Indeed, if a tube is not properly orientated as itpasses into the tool, it is brought into contact with the stop facesduring the closing operation and is thereby aligned in its edgeposition. The result, if the roller bars 88 do not have a floatingmount, is slippage between tube and roller bars 88 about thelongitudinal axis of the tube. The effect of the floating mount is thatscratching of the tube is reliably precluded, and uniform pressingconditions prevail even if the tube, which may be a long one, makesfurther autonomous movements. Hence, the tool closes the gap whilealigning the edges to be welded, this closure being continuous,tapering, and accurately aligned. The individual tools 33 with theirbrackets 34 and adjusting and/or pressure-generating means 35 are freelypositionable around the tube body. This can be seen from FIG. 4, inwhich a round support 27 in the shape of a segment of a circle is shown,which surrounds the body 19 and has a groove 38 on which the individualtools 33 to 35 can be slid and positioned by means of mountings 39. Thenumber of such tools (five) shown in FIG. 4 can of course be varied. Theentire support 27 floats on rollers 29 attached to the machine frame(not shown), so that the support 27 and with it the tools 33 can shiftabout the longitudinal axis of the body 19, or in other words, the toolseffectively have a floating mounting. This is, of course, also the casefor the other support 28 which is shown in FIG. 1 and which supports theother end of the tool 33, so that the entire tool is able to adaptitself to the tube body. Preferably, spring means 40 and 41 are providedwhich whilst allowing rotational shifts of the support 27 (or 28),restore the support 27 and its tools to a basic position, ready for thenext body, after the body 19 exits from the tool.

After leaving the tool which has just been described, the tube body,with its gap already largely closed, passes into the welding toolproper. This is represented schematically in FIG. 1 by the rollers 50,51 and 54, 55. For a more detailed description it will be necessary torefer to FIG. 5. These rollers also have a modular arrangement, witheach ring of rollers forming in itself a module that can be separatelydriven, (as will be explained in detail presently), can be separatelymoved, and can even be separately removed from the welding machine oradded. The roller rings can be arranged in corresponding mountings onthe machine frame and can, e.g., be made to travel on rails tofacilitate their removal or replacement. An edge monitoring unit 43 (seeFIG. 1) which detects the position of the edges, e.g., by an opticaltechnique can be arranged before the welding point. Such a unit forobtaining the correct butted position of two metal sheets is shown in EPPatent Application Serial No. 97109160.8

In accordance with this edge position detection, the welding tool canthen be operated, as will be described later; and/or the welding beam46—which, from a welding beam source 45 (not shown) is brought to bearon the workpiece via a welding beam guide 44 (only schematicallysuggested)—can be influenced.

FIG. 5 shows one module of the welding tool with a roller ring in afrontal view of its feed through opening. The rollers 50 to 55 form theroller ring. The individual rollers are waisted, so that they form afeed through profile matching the body. The rollers 50 and 51 which liehorizontally are driven by motors 58 and 57 in the illustrated example,to convey the body through the welding tool. The rollers 54 and 55 aremounted on carriers 70 and 71 which are pivotable about the pivot axes70′ and 71′ respectively. The pivot axes 70′ and 71′ lie parallel withthe longitudinal axis of the feed through opening formed by the rollers.In the example shown, the carriers 70 and 71 are acted on by springs 75and 76 so that resilient pressure is applied by the rollers 54 and 55 tothe tube body. This resilient pressure causes a closing of the gap onthe opposite side of the body whereby any waviness of the edges can beevened out so as not to exceed the tolerance for the gap-opening at thewelding point. Pressing with the rollers 54 and 55 can, however, causeV-shaped splaying of the gap, so pressure is preferably also exerted onthe body with the rollers 52 and 53 to compensate for this. For thispurpose the rollers 52 and 53 are likewise pivotable on carriers 59 and60 about pivot axes 61 and 62, pivoting preferably being effected bydrives 64 and 63 respectively, which are represented schematically inthe drawing as motors with spindles. The pressure exerted on the bodywith the rollers 52 and 53 can thereby be controlled or regulated and/orapplied in response to the sensor 43 so that a small-sized gap results.In order that damage to the body is reliably precluded when the rollers52, 53 and also 54, 55 are tilted, they are arranged with theirrespective rims 67, 68 and 67′, 68′ lying in the plane which passesthrough the respective pivot axes and the longitudinal axis of the feedthrough opening. Lines denoting these planes are shown as the line 66through the pivot axes 61 and 70′ and the center of the feed throughopening, for the rollers 52 and 55; and the line 65 for the pivot axes62 and 71′. The aforementioned elements of the closing tool are arrangedon an independent support 80 for each module. Each support 80 may beguided, and fixable in its position, in rails or on rollers on themachine frame. The individual modules can be quickly exchanged fordifferent tube body diameters and adjusted by their interfaces, i.e.,support plates 81,82 and spindle 84. The welding tool may close theedges of the bodies completely. As previously stated, it has a modularconstruction in the longitudinal direction with, according torequirements and/or sheet-metal properties, several modular units thatcan have different settings; and modules without pivotable rollers canalso be provided. For example, a module can be used at the weldinglocation to generate a transverse force acting on the edges by pressingdown the edges with the corresponding rollers. It is practicallypossible to produce a nil gap between the edges at the location of thewelding point, and the maximum tolerance of 0.08 mm for laser weldingcan easily be achieved. In particular, cutting variations are evenedout. Instead of the three such modules shown in FIG. 1, more suchmodules can of course be arranged, especially for long tubes and largesheet-metal thicknesses. The welding tool modules also fulfil atransport function for the tube bodies that is independent of theirlength. It may also be advantageous from a transport aspect to connectseveral modules in series, depending on tube length and material. Theindividual modules can be adjustable to different body diameters, assuggested in FIG. 5 by the spindle 84 with which the two halves 81 and82 of the support 80 can be displaced with respect to one another toform feed through openings of different diameters.

A module with driven rollers 50 and 51 can also be arranged after thewelding point. This can be particularly advantageous where an opticalinspection of the quality of the seam is made with a device 86 whichreacts sensitively to variations in the conveyed height of the tube. Thefully welded tube can then be conveyed out of the welding plant andstacked. A control 91 of the welding machine may operate in conjunctionwith the control 90 of the transfer apparatus.

None of the elements that have been described need necessarily beprovided in a welding plant. For example, a run-in device may be omittedif the gap width of the tube as delivered by the transfer unitcorresponds to the width on entry into the tool.

It would also be feasible for example to extend the tool and/or theroller bars 88 so that a tapering closure of the edges culminating in abutted position at the welding point is absolutely guaranteed. A weldingtool proper might then be omitted.

Where, however, tubes of any desired length with properties making themdifficult to weld are to be fabricated in large production runs and withan extremely low incidence of weld defects, an arrangement with run-indevice, tool, welding tool and sensors will secure the desired result interms of the quality of the welded seam, as the inevitable warping inthe position of the edges can then be corrected.

Although this invention has been shown and described with respect to thedetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. Draw-in device characterized in that the devicehas drawing-in means, in particular draw-in rollers driven in rotation,which can be moved, driven to a receiving position to receive a tubebody without making contact with it and can be moved towards each otherby a predetermined amount, in a controlled manner, to contact the bodyin order to draw it in, sensing means for detecting a body, controlmeans responding thereto, and actuating means controlled by the latterfor moving the drawing-in means, being provided.
 2. A draw-in device fora welding plant, comprising: a plurality of draw-in rollers, each drivenin rotation, which can be moved to a first position to receive a tubebody without making contact with it, and can then be moved towards eachother by a predetermined amount to a second position, to contact thebody in order to draw it in; sensing means for detecting the tube body;control means responsive to the sensing means; and actuating meanscontrolled by the control means for moving the draw-in rollers.